Although vitamin D is clearly essential for the regulation of calcium (Ca) and P.sub.i metabolism, its role in reproduction is poorly defined, especially in mammals. During pregnancy and lactation, the rate of synthesis and plasma levels of 1,25-dihydroxyvitamin D.sub.3 (1,25-(OH).sub.2 D.sub.3), the hormonally active form of vitamin D, are increased. Although vitamin D-deficient female rats can reproduce, it is known that the absence of vitamin D diminishes their mating success and fertility as well as litter size and neonatal growth. Thus, vitamin D appears to play a role in female reproduction. However, to date, no attempt has been made to determine if vitamin D is required for reproductive functions of the male. This is particularly important since the receptor for 1,25-(OH).sub.2 D.sub.3 has been found in male reproductive organs.
Thus, in accordance with the present disclosure, it has been found that although vitamin D-deficient male mammals can reproduce, vitamin D deficiency in male mammals markedly reduces their mating success and fertility. Additionally, it has been found that diminished reproductive functions of male and female mammals may be improved dramatically by the administration of vitamin D compounds such as vitamin D.sub.3 and 1,25-(OH).sub.2 D.sub.3, as well as by other compounds having vitamin D-like activity.